The invention is directed toward the field of lithographic printing press dampening control, more particularly to print-based automatic control of dampening across the width of the printing operation, such as the width of a printed web.
Lithographic printing is based on the fact that oil and water don""t mix. An image, typically on an aluminum printing plate, is created by a thin coating of oleophilic (oil-loving) material. Non-image areas are bare aluminum, which are entirely hydrophilic (water-loving). A printing plate rotates, first past some mechanism which applies a water solution called dampening solution to the hydrophilic areas, then to one or more rollers which apply oil-based ink to the remaining areas not repelling the ink by the water solution. Dampening solution, although loosely called water, also contains wetting agents, acids, fungicides and algicides, and often other compounds. The plate, with ink on the desired areas and water on the remainder, now prints ink onto a substrate, typically paper, either directly or more often via an intermediate xe2x80x98blanketxe2x80x99 roller. If several inks are to be applied, the paper may pass through several such xe2x80x98print units,xe2x80x99 applying, for instance, black, cyan, magenta, and yellow inks in sequence, and may apply the inks to both sides of the paper, thus forming printed images and text.
Water is applied to the plate in a variety of ways. A xe2x80x98brushxe2x80x99 system utilizes a rotary brush which is wetted against a roller which dips into a tray containing dampening solution. The brush bristles flick water onto intermediate rollers which wet the plate. A brush system is disclosed in U.S. Pat. No. 6,138,563 with attention to FIG. 6. This system guarantees one-way travel of the water, but is difficult to control and has largely been abandoned. A series of rubber dampening rollers may be used to transfer dampening solution to the plate. Any system which allows two-way water motion, backtracking from the plate back to the tray, risks ink rubbing off the plate and back through the roller train into the tray, contaminating the dampening solution with ink. Examples of such systems may be found in U.S. Pat. Nos. 5,249,036 and 5,957,054. Another method of water application is by a series of spray heads or nozzles across the width of the printing plate; being noncontact, no backtracking of ink is possible. Examples of spray dampeners are found in U.S. Pat. Nos. 4,198,907; 4,649,818; 4,815,375; 4,932,319; 5,025,722; and 5,595,116. A similar system with the advantage of not producing hard-to-control droplets is disclosed in U.S. Pat. No. 6,561,090; this system produces streaming rather than spraying of the dampener. For the purposes of this disclosure, xe2x80x9csprayxe2x80x9d shall be defined to encompass this streaming method or other squirting methods. A pulse-width-modulation system for controlling such spray nozzles is disclosed in U.S. Pat. No. 5,038,681.
Dampener consumption is not consistent across the plate. Consumption depends on ink coverage, ambient temperature which affects evaporation, and absorption rates of the paper being printed, which in turn, depends partially on the moisture content of the paper. The paper is often stored for extended periods prior to use, and the outer portions of the stored bulk paper are more prone to water evaporation or absorption, depending on ambient humidity. Therefore the dampening solution consumption may depend on whether it is near the edge of the paper as stored. On a high speed printing press, such variables may change quickly as a stack or roll of paper is consumed, leading to excess or inadequate dampening.
Excess dampening leads to xe2x80x98emulsificationxe2x80x99 of water into the ink; tiny droplets of excess water mix into the ink on the various ink rollers. The ink is often applied to the substrate in the form of small halftone dots, which tend to be broken up and spread by the intruding water. Spreading of the halftone dots tends to give a darker or stronger color. Areas of solid ink are diluted by the intruding water, giving a weaker color. The resultant image therefore has undesired xe2x80x98flatxe2x80x99 color with poor contrast and a cartoonish look. Such color typically xe2x80x98swimsxe2x80x99 (is inconsistent, with measurements varying with a standard deviation of about 0.1D or greater); xe2x80x98swimmingxe2x80x99 is a convenient mnemonic for pressmen to remember that inconsistent color may be caused by excess water.
The best color, with sharp contrast and consistent dot structure, is provided with a minimum amount of water. However, inadequate dampening leads to a xe2x80x98dryup;xe2x80x99 without a protective water layer, ink will adhere to the plate in undesired locations where no ink belongs. The affected paper is xe2x80x98tintedxe2x80x99 with undesired color where white is desired. Note that both excess water and inadequate water will increase the density of halftone areas; excess water due to the dots being broken up by entrained water, inadequate water due to tinting of the white portion of the halftone. The tint is actually an apparently-random fine pattern, whose pattern is that of the aluminum crystal grain of the plate. If an optical system has adequate resolution to resolve the pattern, desired-white areas will show this grain pattern, which resembles optical noise and will be recognized by a high standard-deviation of brightness levels within the desired-white area. If the dryup is severe, it causes a significant consumption of ink, xe2x80x98robbingxe2x80x99 ink which would otherwise be in the proper printed image. The result is that solid tones of the expected ink are weakened by the loss of ink to the dryup.
The best-known example of inconsistent dampening is dryups at the paper edges of high-speed web presses; heat from the bearings of the various rollers, conducts to the edges of the various rollers, increasing evaporation. Dryups at the edges of the web are a common pressman""s irritation, and control of color at the edge few inches of paper is notoriously unreliable.
To combat edge dryups, dampening rollers may be deliberately misaligned, with the left side canted slightly too high and the right side too low. This provides more xe2x80x98squeezexe2x80x99 of the roller against the center of the next roller in the roller train, and more water removal at the center, with more water at the edges, preventing dryups. Similarly, a roller may be pressed tighter on one side than the other, to give more dampening on the needed side. Such mechanical adjustment is slow, manpower-intensive, and error-prone guesswork. Spray dampeners often have individual control of each spray head, a more convenient adjustment.
A limitation of these systems is that they lack feedback to control dampening according to the actual resultant printing, and do not recognize that needed dampening varies across the width of the paper. Feedback according to speed is disclosed in U.S. Pat. No. 6,138,563, and feedback according to water on the plate is disclosed in U.S. Pat. Nos. 5,520,113 and 6,138,563, but these are merely secondary indicators of the actual print quality. Online measurement of the actual resultant printing is disclosed in U.S. Pat. Nos. 5,791,249 and 6,058,201.
There exists a need for a control system which additionally controls dampening in a variable manner across the width of the paper, controlled by the actual resultant print.
The preferred form of the instant invention measures the printed ink in unprinted, partial-tone (meaning halftone dots partially covering the paper, not necessarily 50%), and full-tone areas in a multiplicity of zones (often called xe2x80x98alleysxe2x80x99 or xe2x80x98key areasxe2x80x99) across the width of the substrate to determine optimum dampening, and controls a corresponding multiplicity of dampening flow controls to provide an optimum quantity of dampener for each zone.